U.S. patent application number 12/445312 was filed with the patent office on 2010-03-25 for flat and thin led-based luminary.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Willem L. Ijzerman, Michel C.J.M. Vissenberg.
Application Number | 20100073925 12/445312 |
Document ID | / |
Family ID | 39015863 |
Filed Date | 2010-03-25 |
United States Patent
Application |
20100073925 |
Kind Code |
A1 |
Vissenberg; Michel C.J.M. ;
et al. |
March 25, 2010 |
FLAT AND THIN LED-BASED LUMINARY
Abstract
A light emitting device is provided, comprising a light guide
plate (100) and at least one light emitting diode (110). An array
of mutually spaced apart reflective surface elements (103, 103') is
arranged between said back surface (102) and said front surface
(103). The reflective surface elements (103, 103') are non-parallel
relative to said back surface (101) and front surface (102), and
have a front side (104) facing said front surface (102) and a back
side (105) facing said back surface (101). The back surface (105)
of a reflective surface element (103) faces the front surface (104)
of an adjacent reflective surface element (103'). The at least one
light emitting diode (110) is arranged to emit light into a region
(111) between two adjacent ones of said reflective surface elements
(103, 103'). Since the reflective surface elements are located
within the light guide, both the back surface and the front surface
of the light guide plate may be made flat. This flat surface may
easily be kept clean from dust and dirt.
Inventors: |
Vissenberg; Michel C.J.M.;
(Eindhoven, NL) ; Ijzerman; Willem L.; (Eindhoven,
NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
EINDHOVEN
NL
|
Family ID: |
39015863 |
Appl. No.: |
12/445312 |
Filed: |
October 12, 2007 |
PCT Filed: |
October 12, 2007 |
PCT NO: |
PCT/IB07/54165 |
371 Date: |
November 10, 2009 |
Current U.S.
Class: |
362/235 ; 257/98;
257/99; 257/E33.067 |
Current CPC
Class: |
G02B 6/0021 20130101;
G02B 6/0031 20130101; G02B 6/0015 20130101; G02B 6/0035 20130101;
G02F 1/133603 20130101; G02F 1/133605 20130101; H01L 33/60
20130101; G02B 6/0011 20130101 |
Class at
Publication: |
362/235 ; 257/98;
257/99; 257/E33.067 |
International
Class: |
F21V 1/00 20060101
F21V001/00; H01L 33/00 20100101 H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 16, 2006 |
EP |
0612231.0 |
Jan 11, 2007 |
EP |
06112437.6 |
Jan 11, 2007 |
EP |
07100360.2 |
Jan 11, 2007 |
EP |
07100361.0 |
Jan 30, 2007 |
EP |
07101366.8 |
Claims
1. A light emitting device, comprising a light guide plate (100)
and at least one light emitting diode (110), which plate (100)
comprises a back surface (101), an opposing front surface (102) and
an array of a plurality of mutually spaced apart reflective surface
elements (103, 103') arranged between said back surface (102) and
said front surface (103), wherein said reflective surface elements
(103, 103') are non-parallel relative to said back surface (101)
and front surface (102), and have a front side (104) facing said
front surface (102) and a back side (105) facing said back surface
(101), the back surface (105) of a reflective surface element (103)
faces the front surface (104) of an adjacent reflective surface
element (103'), and said at least one light emitting diode (110) is
arranged to emit light into a region (111) between two adjacent
ones of said reflective surface elements (103, 103').
2. A light emitting device according to claim 1, wherein said
reflective surface elements (103, 103') are separated from said
front surface (102).
3. A light emitting device according to claim 1, wherein said
reflective surface elements (103, 103') are mutually coplanar.
4. A light emitting device according to claim 1, wherein at least
one of said reflective surface elements (103) comprises a slot
(106) between said front side (104) and said back side (105)
thereof.
5. A light emitting device according to claim 1, wherein a mirror
(107) is arranged at one or more of, or between, said front side
(104) and said back side (105) of said reflective surface elements
(103, 103').
6. A light emitting device according to claim 1, wherein said at
least one light emitting diode is arranged in a recess (109) in
said back surface (101) located between said two adjacent
reflective surface elements (103, 103').
7. A light emitting device according to claim 1, wherein said at
least one light emitting diode (110) is in optical contact with
said light guide plate (100).
8. A light emitting device according to claim 1, wherein said at
least one light emitting diode (110) is molded into said light
guide plate (100) between said two adjacent reflective surface
elements (103, 103').
9. A light emitting device according to claim 1, wherein a
reflective surface (108) is arranged at said back surface
(101).
10. A light emitting device according to claim 1, wherein more than
one light emitting diode (110, 110') emits light into a single
space between two adjacent reflective surface elements (103,
103').
11. A light emitting device according to claim 1, wherein the front
side (104) of said reflective surface elements (103, 103') forms an
angle in the range of from 1.degree. to 20.degree. to said front
surface (102).
12. A light emitting device according to claim 1, wherein at least
one collimator (220) is formed in a space between two adjacent
reflective surface elements (103, 103'), which collimator (220) is
arranged in the light path between a light emitting diode (110)
emitting light into said space, and to collimate light at least in
the direction in the plane of said light guide and perpendicular to
first direction (L), in which direction said array of reflective
surface elements (103, 103') extends.
13. A light emitting device according to claim 12, wherein said
collimator (220) is funnel-shaped.
14. A light emitting device according to claim 12, wherein at least
two collimators (220, 220') are formed side by side in said region
(111) between said two adjacent surface elements (103, 103'), and
wherein said two collimators (220, 220') are separated by an open
void (222).
15. A light emitting device according to claim 1, wherein a
redirection foil (300) is arranged at said front surface of said
light guide, said redirection foil having a prism-faced surface
(301) facing said front surface (102).
16-17. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a light emitting device
comprising at least one light emitting diode and a light guide
plate. The present invention also relates to such a light guide
plate it self and a luminary comprising at least one such light
emitting device.
BACKGROUND OF THE INVENTION
[0002] Especially if applied in, for instance an office or a
professional environment, luminaries should fulfill several
requirements. Firstly, the light source should have a sufficiently
long lifetime. Conventional luminaries are often based on
fluorescent tubes, which have a relatively limited lifetime. In a
typical office environment, the tubes themselves need to be
replaced every 6000 hours. This corresponds to a replacement every
2 years, which adds to the cost of ownership.
[0003] Secondly, the light output of the luminary should be robust
against dust and other dirt. A luminary that collects dust will
become less efficient, since the dirt blocks light. Since cleaning
the luminary is an expensive matter, the design should be robust
against dust and dirt.
[0004] Thirdly, the luminary should satisfy an anti-glare
requirement (i.e. the unified glare ratio should be sufficiently
small). This anti-glare requirement means that the luminary should
not show any bright spots. In particular, there should be no bright
spots if the luminary is viewed under an oblique angle.
[0005] A luminary of the prior art is disclosed in U.S. Pat. No.
6,241,358, describing a lighting panel consisting of a set of light
guide blocks in tandem arrangement, where a separate fluorescent
tube provide light for each light guide block. The light from the
fluorescent tubes is transmitted into the respective light guide
block, is distributed therein and is transmitted through an output
surface of the light guide block. However, as mentioned above,
fluorescent tubes have a limited lifetime and are expensive to
replace. Further, the breakdown of a single fluorescent tube in
this prior art luminary has a drastic negative impact on the
lighting capacity of the lighting panel and on the homogeneity of
the light from the lighting panel. Thus, when one of the tubes
breaks down, it will be necessary to replace this broken tube
immediately.
[0006] Further, the saw tooth shaped backside of the luminary
according to the '358 patent can easily trap a lot of dust and
dirt, and is rather complicated to clean.
[0007] Additionally, fluorescent tubes emit a constant spectrum,
which limits the color variability capacity of such a lighting
panel.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to at least partly
overcome the problems of the prior art and to provide a light
emitting device that has a long lifetime, is resistant to dirt and
dust and is capable of emitting light of variable colors.
[0009] The present inventors have found that the above objects may
be achieved by means of a light emitting device accorded to the
appended claims. Thus, in a first aspect, the present invention
relates to light emitting device, comprising a light guide plate
and at least one light emitting diode.
[0010] The light guide plate comprises a back surface, an opposing
front surface and an array of mutually spaced apart reflective
surface elements arranged between said back surface and said front
surface.
[0011] The reflective surface elements are non-parallel relative to
said front surface and said back surface, such that each has a
front side facing said front surface and a back side facing said
back surface. The reflective surface elements are further arranged
such that the back side of one reflective surface element faces the
front side of an adjacent reflective surface element.
[0012] The at least one light emitting diode is arranged to emit
light into a region between two adjacent ones of said reflective
surface elements.
[0013] The light from the LED will be received into the light guide
plate and will be distributed therein before exiting the light
guide plate via the front surface thereof. The light will be guided
from the back surface to the front surface of the light guide by
passage between and reflections on two adjacent reflective surface
elements.
[0014] Due to that the front surface of the light guide and the
front side of the reflective surface elements forms an angle, the
light will eventually be incident on the front surface at an angle
below the critical angle for total internal reflection and will be
coupled out of the light guide plate.
[0015] Since the reflective surface elements are located within the
light guide, both the back surface and the front surface of the
light guide plate may be made flat. This flat surface may easily be
kept clean from dust and dirt. The use of light emitting diodes as
primary light sources is advantageous as they have a long lifetime.
Hence, service intervals will be extended, leading to a lower cost
of ownership.
[0016] Further, light emitting diodes are capable of emitting light
of saturated colors, allowing the light emitting device to produce
light with high color-variability.
[0017] In embodiments of the present invention, the reflective
surface elements are separated from said front surface.
[0018] When the reflective surface elements are separated from,
i.e. not connected to, the front surface of the light guide, this
leaves an opening, a gap, between the front surface and the
reflective surface elements. Hence, light from an LED which does
not couple out of the light guide plate at its incidence on the
front surface (for example due to that the angle of incidence
exceeded the critical angle for total internal reflection), can be
reflected into the region between another, adjacent, pair of
reflective surface elements.
[0019] In embodiments of the present invention, the reflective
surface elements are mutually essentially coplanar.
[0020] In order to obtain an essentially homogenous light from a
light emitting device of the present invention that has an
essentially rectangular shape, it is preferred that the reflective
surface elements are parallel, or coplanar, such that the light
from each on the LEDs are directed in the same manner
[0021] In embodiments of the present invention at least part of
said reflective surface elements may comprise a slot between said
front side and said backside thereof.
[0022] A slot between the front and the backsides of the reflective
surface element allows total internal reflection to take place on
these elements. This will increase the light utilization, since
reflection can take place without any absorption of light.
[0023] In embodiments of the present invention, a mirror may be
arranged at one or more of, or between, said front side and said
back side of said reflective surface elements
[0024] For designs of the light guide plate, where a significant
portion of the light incident on the front and/or the back side of
the reflective surface elements is below the critical angle of
incidence, a mirror may be arranged such that also this light is
reflected back into the space between two adjacent reflective
surface elements.
[0025] In embodiments of the present invention, at least one of
said light emitting diodes may be arranged in a recess in said back
surface located between said two adjacent reflective surface
elements.
[0026] In order to have an essentially flat back surface of the
total light emitting device, the light emitting diodes may be
arranged in recesses in the back side of the light guide plate.
This gives a mechanically robust design since the LEDs are less
exposed for mechanical wear and tear. Further, the LEDs can easily
be physically fixated in the light guide, which may obviate the
need for a separate PCB circuit board on which the LEDs typically
are arranged. Instead, the LEDs may be connected by simple and
cheap electrical wiring on the backside of the device.
[0027] In embodiments of the present invention, said at least one
light emitting diode may be in optical contact with said light
guide plate.
[0028] When the LEDs are in optical contact with the light guide
material, the light utilization efficiency is increased since a
larger portion (essentially all) of the light emitted by the LEDs
is coupled into the light guide, in comparison to when the LEDs are
located at a distance from and/or not in optical contact with the
light guide material. However, the light will be received into the
light guide plate with a angular spread of up to 90.degree. with
respect to the general direction of the received light, and will
hence not be guided by total internal reflection in the light
guide. The reflective surface elements in the light guide plate
will provide the needed collimation in the direction along their
extension
[0029] In embodiments of the present invention, said at least one
light emitting diode may be molded into said light guide plate
between said two adjacent reflective surface elements.
[0030] When the light emitting diodes are molded into the light
guide, they are physically fixated by the light guide plate and can
be essentially non-exposed to the surroundings, giving a very
robust design, which is easily kept clean.
[0031] Further, by molding the LEDs into the light guide plate,
they become in optical contact to the light guide plate.
[0032] In embodiments of the present invention, a reflective
surface may be arranged at said back surface.
[0033] Light propagating in the light guide plate may be incident
on the back surface at an angle below the critical angle for total
internal reflection, and may thus be coupled out from the light
guide via the back surface. This is generally unwanted, and a
reflective surface may be used on the back side of the light guide
plate in order to reflect such outcoupled light back into the light
guide for eventual out-coupling via the front surface thereof.
[0034] In embodiments of the present invention, more than one light
emitting diode may emit light into a single region between two
adjacent reflective surface elements.
[0035] A plurality of LEDs arranged to emit light into a single
such region may together form an extended light source. Such an
extended light source will not fully be non-functional in the case
one or a few of the LEDs in that plurality of LEDs break down,
since the neighboring LEDs will still be in operation. Hence, this
yields a robust design for a light emitting device.
[0036] Further, a plurality of LEDs of different colors, typically
independently addressable, may be used in such a single region in
order to provide a color variable light emitting device.
[0037] In embodiments of the present invention, the reflective
surface elements may form an angle of from about 1.degree. to about
20.degree. to said front and back surface. The angle is set such
that the desired degree of collimation is achieved.
[0038] When this angle is between about 1.degree. and about
20.degree., preferably between 2.degree. and 10.degree., the light
from each light emitting diode is distributed in the light guide
such that it is outcoupled from the front surface of the light
guide at locations corresponding to several pairs of adjacent
reflecting surface elements, such that the light from such a light
emitting device would be very homogenous over the whole light guide
plate.
[0039] In embodiments of the present invention, at least one
collimator may be formed in a region between two adjacent
reflective surface elements, which collimator is arranged in the
light path between a light emitting diode emitting light into said
space, and to collimate light at least in the direction in the
plane of said light guide and perpendicular to the first direction,
in which the array of reflective surface elements is extending.
[0040] The reflective surface elements in the light guide will
essentially not provide any collimation of light in the direction
perpendicular to said first direction. By arranging collimators in
the regions between adjacent reflective surfaces such that the
light in the light guide is collimated in the direction in the
plane of the light guide and perpendicular to the extension of the
array of reflective surface elements, the light exiting the light
guide plate will be collimated in this direction. Such collimators
are typically funnel-shaped.
[0041] In embodiments of the present invention, at least two
collimators may be formed side by side in a space between said two
adjacent surface elements, wherein said two collimators are
separated by an open void.
[0042] When the collimators are made in the material of the light
guide plate, and the space between adjacent collimators are formed
by an open void, total internal reflection is possible in the
interface between the light guide material of the collimator and
the open void. Hence, loss-less reflection can take place in these
interfaces, increasing the light utilization efficiency of the
device.
[0043] In embodiments of the present invention a redirection foil
may be arranged at said front surface of said light guide, said
redirection foil having a prism-faced surface facing said front
surface.
[0044] In a second aspect, the present invention also relates to a
light guide plate as describes above, as such.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] This and other aspects of the present invention will now be
described in more detail, with reference to the appended drawings
showing a currently preferred embodiment of the invention.
[0046] FIG. 1a illustrates one embodiment of a light emitting
device of the present invention.
[0047] FIG. 1b illustrates a detail of FIG. 1a.
[0048] FIG. 1c illustrates in perspective view, the embodiment of
FIG. 1a.
[0049] FIG. 2 illustrates another embodiment of a light emitting
device of the present invention.
DETAILED DESCRIPTION
[0050] The present invention relates in one aspect to a light
emitting device comprising at least a light guide plate and at
least one light emitting diode arranged to emit light into the
light guide plate. In another aspect, the present invention relates
to such a light guide plate it self. While the below embodiments
describe a light emitting device, all details regarding the light
guide plate in the described light emitting device also applies to
the light guide plate aspect of the invention.
[0051] An exemplary embodiment of a light emitting device of the
present invention is illustrated in FIG. 1a-c and comprises a light
guide plate 100 and a plurality of light emitting diodes 110.
[0052] The light guide plate 100 is at least partly transmissive,
e.g. translucent or even transparent, and e.g. made of a
transparent material, such as optically clear glass, ceramics or
plastic material. PMMA (polymetylmethacrylate) and polycarbonate
are examples of suitable plastic materials.
[0053] The light guide plate 100 is an essentially flat plate
having a back surface 101 facing the interior of the light emitting
device and a front surface 102, which faces the viewer of the
device.
[0054] In the light guide plate 100, a plurality of mutually spaced
apart reflective surface elements 103 are arranged. The array
extends in a direction L in the plane of the light guide plate
100.
[0055] Typically, the reflective surface elements 103 represent
flat and relatively thin (i.e. thin, in comparison to the thickness
of the light guide plate) surfaces.
[0056] As seen in FIG. 1c, The reflective surface elements 103 are
extended in the direction in the plane of the light guide plate 100
perpendicular to the direction L of the array. Further, the
reflective surface elements 103 are connected to the back surface
101, but in this embodiment separated from the front surface 102.
Hence, in the reflective surface elements extend from the back
surface but not fully to the front surface of the light guide
plate. Hence, in the direction of the thickness of the light guide
plate 100 (i.e. from the back surface to the front surface), the
light guide plate is divided into two portions: a lower portion
towards the back surface 101, where the reflective surface elements
103 are located, and an upper portion, towards the front surface
102, where no such reflective surface elements 103 are located.
[0057] The reflective surface elements 103 are further non-parallel
relative to the front and back surfaces of the plate, such that the
elements have a front side 104 facing the front surface 102 of the
light guide plate 100, and a back side 105 facing the back surface
101 of the light guide plate 100, and the elements are arranged
such that the back side 105 of one element 103 faces the front side
104 of an adjacent element 103'.
[0058] The array of reflective surface elements 103 may be a linear
array, typically in a linear (e.g. rectangular) light guide plate.
In such a linear array, the reflective surface elements 103 are
typically mutually essentially coplanar. However, the present
invention is not limited to this, and the array may be a circular
array, such as for use in a circular light guide plate (in which
case the direction L of the array follows the perimeter of the
circular light guide). Alternatively, the angle between the
reflective surface element 103 and the front and back surfaces of
the light guide may vary between different surface elements
103.
[0059] The light emitting diodes 110 are arranged to emit light
into regions 111 formed between two adjacent reflective surface
elements 103, 103', such that the region 111 is located between the
back surface 105 of one of the elements 103 and the front surface
104 on the other one of the two elements 103'.
[0060] The light received into such a region 111 is propagated
towards the front surface 102 of the light guide by reflection on
at least one of the reflective surface elements 103, 103' and/or
optionally by reflection on the portion of the back surface 101
that is located in said region, or by passing directly to the front
surface without reflection.
[0061] The region 111 forms a first wedge shaped portion between
the reflective surface element 103 and the back surface 101 of the
light guide plate 100. This first wedge portion collimates the
light in the direction of the height of the light guide plate.
[0062] This wedge portion between the reflective surface element
103 and the back surface 101 of the light guide plate is used to
collimate any light along the direction L from angles smaller than
the critical angle for TIR (total internal reflection) (with
respect to the front surface 102) to angles that are larger than
the critical angle for TIR (this may occur when the LED is in
optical contact with the guide, or when the LED light is not aimed
along direction L). Thus, the light is captured into the light
guide by this first wedge portion.
[0063] The remaining part of region 111 forms an opposite, second
wedge shape between the adjacent reflective surface element 103'
and the front surface 102. Instead of collimating, this second
wedge de-collimates the light and causes the light to leak out of
the light guide at surface 102.
[0064] Light that is incident on the front surface 102 of the light
guide plate 100 at an angle below the critical angle of total
internal reflection (TIR) is coupled out of the light guide plate.
However, a portion of the light emanating from an LED will be
incident on the front surface at angles above the critical angle
for TIR, and will thus be reflected towards the back surface of the
light guide plate.
[0065] Since, in the presently described embodiment, there is a gap
between the front surface 102 and the reflective surface elements
103, (i.e. the above mentioned top portion), light from one LED,
originally passing in a certain region 111 between a pair of
adjacent reflective surface elements 103, 103', may be reflected on
the front surface into another region 111', formed between another
pair of reflective surface elements 103', 103''.
[0066] The light that is reflected on the front surface 102 back
into the light guide plate will eventually be reflected on the
front side 104 of a reflective surface element once more towards
the front surface 102. Since the front side 104 of the reflective
surface elements 103 forms an angle towards the front surface 102,
the subsequent angle of incidence on the front surface 102 will
differ from the previous angle of incidence, and the light will
propagate in the light guide until it eventually is coupled out of
the light guide.
[0067] The opening between the front surface and the reflective
surface elements allows the light from one LED to be distributed
over a large portion of the light guide.
[0068] The reflective surface element may comprise a reflecting
mirror 107 to effect the reflection, or may, as is illustrated in
FIG. 1b, being detail from FIG. 1a, optionally comprise a slot
between the front side 104 and the back side 105 of the element
103.
[0069] Such a slot 106 should typically be substantially wider
(i.e. the distance from the front side to the back side of the
reflective surface element) than the wavelength of the light to be
propagated in the waveguide. Further, such a slot 106 is typically
empty (air) or is filled with a material having a refractive index
substantially lower than the refractive index of the light guide
plate material. Thus, total internal reflection is allowed on the
sides 104, 105 of the reflective surface element 103, yielding
essentially loss-less reflection.
[0070] A reflecting mirror 107 may be arranged within the slot 106,
such as between, or on one of, the front side or back side of the
surface element such as to reflect also light incident on the
reflective surface element at an angle of incidence below the
critical angle of total internal reflection.
[0071] Each light emitting diode (LED) 110 are arranged to emit
light into a region 111 between two adjacent reflective surfaces
103, 103'. The LEDs 110 may be arranged on or emitting onto the
back surface 101 of the light guide plate 100. Typically, the LEDs
110 are accommodated in recesses 109 arranged in the back surface
101 or are molded into the back surface 101 of the light guide
plate.
[0072] By molding the LED 110 into the light guide 100, the LED is
in optical contact with the light guide and the light is
efficiently coupled into the light guide.
[0073] As illustrated in FIG. 1c, a plurality (two or more) of LEDs
110, 110' may be arranged to emit light into the same single region
111 between two adjacent reflective surface elements 103, 103'.
These two or more LEDs 110, 110' are arranged side by side to form
a row extending along the extension in the plane of the light guide
and perpendicular to the extension L of the array of reflective
surface elements 103, 103'. The row of LEDs 110, 110' may act as an
extended linear light source.
[0074] A row of LEDs 110, 110' may comprise LEDs emitting light of
different color. For example, such a row may comprise one or more
sets, each set comprising for instance a red, green and blue LED,
for forming a color variable light emitting device.
[0075] All types of LEDs may be used in a light emitting device of
the present invention, including, but not limited to side-emitting
and top-emitting LEDs, inorganic, organic and polymeric based LEDs,
and LEDs emitting light in the visible, UV and IR wavelength range
of light.
[0076] The electrical and thermal connections to the LEDs may be
either via the bottom side 101 or via the reflectors 103. The LEDs
may also be mounted on the reflective surface elements 103, and
then together molded in the light guide plate 100.
[0077] Some light may non-intentionally be coupled out of the light
guide plate via the back surface 101 thereof. A reflective surface
108 may be arranged on the back side of the back surface 101 in
order to reflect such light back into the light guide plate for
increased light utilization efficiency.
[0078] This surface 108 may contain holes for electrical wiring or
thermal contact to the LEDs.
[0079] The distance between two adjacent reflective surface
elements 103, 103', counted along the extension L of the array of
elements (i.e. the pitch of the array) is typically in the order of
one or a few centimeters, such as from about 0.5 to 30 cm,
typically from about 1 to about 10 cm.
[0080] For currently used low-power LEDs, such a pitch of about 1-2
cm, and for currently used high-power LEDs, such a pitch of about
3-6 cm has proven useful. However, with increasing LED efficiency
and lumens/package, this pitch may increase in the future.
[0081] Along the extension L of the array, the pitch may be
constant, or may alternatively vary.
[0082] The thickness of the light guide plate (i.e. the distance
from the back surface to the front surface thereof) is typically in
the order of a few millimeters, such as from about 1 to about 20
mm, typically from about 2 to about 10 mm. The thickness however
depends on the height of the LEDs and pitch of the reflective
surface elements. LEDs are getting smaller, which would allow for
thinner light guides. On the other hand, the amount of light per
package is increasing, and therefore the pitch could increase.
Hence, the thickness could be both smaller and higher than the
above mentioned.
[0083] Typically, the height of the reflective surface elements
(counted along the thickness of the plate) is from about 10 to 100%
(i.e. from the back to the front surface) of the thickness of the
plate, typically from about 30 to about 70%.
[0084] When the reflective surface elements 103 extend fully (100%)
from the back surface 101 to the front surface 102, there will be
no mixing of light from LEDs arranged in adjacent regions 111,
111', but only mixing of light from LEDs arranged in the same
space.
[0085] The angle between the front surface 102 of the light guide
100 and the front side 104 of the reflective surface elements 103
is typically in the order of a few degrees, such as from about 2 to
about 20.degree., typically in the range of from about 4 to about
10.degree..
[0086] In general, a smaller angle gives a better collimation.
However, given the LED height (for example when the LED is molded
into the back surface of the light guide plate) and the pitch, the
LEDs will no longer fit between the reflective surface element and
the back surface of the light guide plate when the angle is too
small. In one exemplary embodiment showing good results, the pitch
of the array of reflective surface elements is 1.85 cm, the
thickness of the light guide plate is 3 mm, the reflective surface
elements extend to 2 mm of the height (66.7%), and the angle of the
reflective surface elements is 6.degree. to the front surface of
the light guide plate.
[0087] Typically, the extension of the reflective surface elements
103, counted along the direction L of the array, is about the same
as the distance between adjacent reflective surface elements,
counted along the same direction (i.e. the pitch). However, this
extension may be somewhat larger than the pitch (resulting in
overlapping reflective surface elements), or may alternatively be
somewhat smaller than the pitch.
[0088] The above-proposed design of the light guide plate does not
yield any essential collimation in the direction in the plane of
the light guide and perpendicular to the extension L of the array
of reflective surface elements 103.
[0089] Hence, in a second exemplary embodiment of a light emitting
device of the present invention, as illustrated in FIG. 2,
collimators 220, to collimate the light in this direction, are
incorporated in the light guide plate design.
[0090] The collimators 220 are arranged in front of the LEDs 110
(i.e. in the beam path between the LEDs and the front surface of
the light guide plate, and in the space between the two adjacent
reflective surface elements (103, 103').
[0091] The collimators are limited by the back side 105 of a
reflective surface element 103, the back surface 101 of the light
guide plate and by side walls 221 extending from the back surface
101 of the light guide plate to the back side 105 of the reflective
surface element
[0092] The collimators 220 are funnel shaped such that the distance
between the sidewalls 221 increases with the distance from the LED.
The sidewalls 221 are typically reflective.
[0093] When light in a collimator 220 is reflected on the sidewalls
221 thereof, the angular spread of the light, in the direction in
the plane of the light guide and perpendicular to the extension L
of the array of reflective surface elements 103, is decreased, i.e.
the light is collimated in this direction.
[0094] Typically, a plurality (more than one) of collimators 220,
220' are located adjacently side by side in the same region 111
between two adjacent reflective surface elements 103, 103' to
collimate the light from separate LEDs 110, 110' or separate groups
of LEDs.
[0095] One way to form the sidewalls 221 is to arrange a void 222
separating the adjacent collimators 220, 220'. The void 222 should
be empty (air, vacuum, other gas) or filled with a material having
refractive index substantially lower than that of the light guide
plate material. The sidewalls 221 are then formed as the interface
between the light guide material and the void 220, and thus, total
internal reflection is possible on the sidewalls 221, leading to
essentially loss-less reflection on these surfaces.
[0096] In addition to providing very efficient reflection, the
collimators 220, 220' may be easily formed in an injection-molding
step.
[0097] Alternatively, mirror surfaces may be molded into the light
guide plate material to form the sidewalls 221.
[0098] The introduction of such collimators 220 into the light
guide plate design yields a light emitting device which provides
light that is collimated in all directions in the plane of the
light guide plate.
[0099] As will be apparent to those skilled in the art, the light
from a light emitting device as illustrated in FIGS. 1 and 2 will
typically exit the light guide plate via the front surface 102
thereof into the surroundings at an noticeable angle with respect
to the normal of the front surface 102.
[0100] For instance, such a light emitting device may be well
suited for illuminating the ceiling when hung on a wall, or for
illuminating a wall when arranged in the ceiling, but also for
other purposes where light emission out of the normal of the front
surface is desired.
[0101] However, in certain applications, it is desired to redirect
the light exiting the light guide plate, for example to obtain
light having a main direction at or close to the normal of the
front surface of the light guide plate.
[0102] Thus, in embodiments of the present invention, and as is
illustrated in FIG. 1a, a redirection sheet 300 may be arranged at
the front surface 102 to receive light that exits the light guide
plate 100 via the front surface 102, in order to redirect the main
direction of this light.
[0103] An example of such a redirection sheet 300 comprises a sheet
of a translucent material (i.e. plastic, ceramic or glass), which
has a prismatic surface 301 facing the front surface 102 of the
light guide plate 100.
[0104] In an embodiment, the prismatic surface 301 comprises an
array of mutually parallel protrusions 302. For high efficiency,
the protrusions 302 are advantageously essentially parallel to the
reflective surface elements 103 the light guide plate 100.
[0105] Typically, the protrusions 302 have a triangularly shaped
cross-section with an apex angle in the range of from 20 to
70.degree., preferably about 40.degree.. The protrusions 302
typically formed at a pitch (distance between two adjacent
protrusions) that are markedly lower than the pitch of the
reflective surface elements 103. Typically, the pitch of the
protrusions 302 is in the range of about 50 to 500 .mu.m.
[0106] The protrusions 302 may further be asymmetric to the normal
of the front surface 102 in order to direct the light from the
light guide plate into a mean direction along the normal of the
front surface. For an asymmetric protrusion 302, the centerline of
the protrusion is not parallel to the normal of the front surface
102.
[0107] Light emitting devices and light guide plates of the present
invention may be used in any application where they are suitable.
Non-limiting exemplary areas of use include the use in or as
luminaries for general lighting applications, such as in homes,
offices, vehicles etc, and for backlighting of display devices. For
example, such a luminary may comprise one or more light emitting
devices of the invention.
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